Three series of experiments are in progress. 1) We propose to quantitate the stability of respiration in the normal new-born infant, in the premature infant and in infants with known cardiac or other defects by obtaining stretches of respiratory tracings obtained by impedance pneumography. The analog tracings are digitised and stored on the disc of a computer for subsequent processing. We aim to determine oscillations imposed on the frequency and depth of breathing by fourier and spectral analysis and to correlate these with the predicted chemical transit times (including the receptor response times) between lung, peripheral and central receptors. 2) We propose in mature foetal sheep to study how it is that for lengthy periods the respiratory rhythm generator is silenced and isolated from most afferent inputs. We have found that during foetal apnoea, activity in units in the reticular formation in medulla and pons is greatly reduced and non-respiratory reflexes e.g., the jaw closing reflex, is totally inhibited. This suggests that the inhibition is central and future studies will be designed to determine the site and nature of this inhibition. 3) We have observed that the respiratory response (phrenic nerve discharge) of new-born lambs to a rise in PaCO2 is greater than in the mature foetus. Two possibilities exist: either the chemoreceptors are less sensitive in the foetus or the stimulus is in some way attenuated. Studies in the next year are designed to distinguish between these possibilities.